Power amplifiers (PAs) that use load line modulation techniques to improve overall amplifier efficiency have been known for some time. Two well-known examples are in the form of a two-way Doherty amplifier, illustrated in FIG. 1, and a Chireix outphasing amplifier, illustrated in FIG. 2. A two-way Doherty amplifier comprises two amplifier stages 101, 102, a first of these being a peak amplifier 101 and a second being a main amplifier 102. The peak amplifier 101 amplifies a phase-shifted version of the input signal, while the main amplifier 102 amplifies an unshifted version. A combiner stage 103 combines the output signals from the amplifier stages 101, 102 and provides an output amplified signal to a load 104. A Chireix outphasing amplifier operates according to a similar principle, with two amplifier stages 201, 202 providing amplified versions of the input signal to a combiner stage 203, which combines the outputs to provide an amplified output signal to a load 204.
N-way Doherty power amplifiers, where N>2, are also known, one example being a three-way Doherty power amplifier, an example of which is illustrated in FIG. 3. In this type of Doherty amplifier, outputs from a main amplifier 301 and two peak amplifiers 302a, 302b are combined in a combiner stage 303 to provide an amplified output signal to a load 304. The arrangement of phase shifts on the input and output stages of the amplifier can be varied. WO 2009/081341 discloses further alternative examples of such amplifiers, one of which is illustrated in FIG. 4, with a similar arrangement of amplifier stages 401, 402a, 402b but with a different arrangement of phase shifts on the inputs to the amplifier stages and in the combiner stage 403.
All of the above described power amplifier concepts share one similarity, which is the use of an output stage power combiner 103, 203, 303, 403 in various different arrangements and with single or multiple λ/4 lines.
A λ/4 (i.e. quarter wavelength) line, when used as combiner, has some limitations related to its frequency properties that can adversely impact the power amplifier, in particular by reducing to a certain extent the operational frequency bandwidth. With increasing complexity of the power combiner this effect becomes more pronounced.
Contemporary power amplifiers of the above type are typically used to amplify digitally modulated signals with a high peak to average ratio (PAR). The efficiency of the power amplifier at moderate power back off levels determines the overall amplifier performance. Based on typical signal statistics, most of the time such power amplifiers will operate with a value of output load significantly deviating from a nominal load of 50 Ω. For instance the main amplifier stage of a 2-way symmetric Doherty power amplifier, when amplifying digitally modulated signals, will tend to experience dynamic excursions of the load that may change from the nominal value to two times the nominal value. For a Chireix outphasing amplifier the load variations tend to be even larger. It is of significant importance that the load conditions do not change with frequency in order to preserve the optimum power amplifier performance over the entire frequency band of operation.
FIG. 5 shows the impedance transformation properties of a λ/4 line, as used as 2-way Doherty combiner centred at 2 GHz. When the λ/4 line transforms the load from the nominal value R1 to the double load 2×R1 the transformation is exact only at the centre frequency. The transformation bandwidth at a voltage standing wave ratio of 1.1:1 is from 1.85 GHz to 2.25 GHz, or a relative bandwidth of 20%. The deviation from the desired load impedance with the change of the operation frequency effectively compromises all important power amplifier parameters such as gain, output power and efficiency.
It is an object of the invention to address one or more of the above mentioned problems.